isProperUpperBound
Given a bound of the form α <: T, we say T is a proper upper bound of α.
Return empty if it is not a proper upper bound. Otherwise it returns the variable of which this is an proper upper bound.
Given a bound of the form α <: T, we say T is a proper upper bound of α.
Return empty if it is not a proper upper bound. Otherwise it returns the variable of which this is an proper upper bound.